The present invention relates to fairings for cables, and is more particularly but not exclusively concerned with fairings for lead-in cables used for towing arrays of seismic streamers during the performance of marine seismic surveys.
In order to perform a marine seismic survey, an array of seismic streamers, each typically several thousand metres tong, is towed at a speed of about 5 knots behind a seismic survey vessel. The streamers contain arrays of hydrophones and associated electronic equipment, distributed along their length. The survey vessel also tows one or more seismic sources, for example, air guns. Acoustic signals produced by the seismic sources are directed down through the water into the earth beneath, where they are reflected by the various strata. The reflected signals are received by the hydrophones, digitised and transmitted to the seismic survey vessel, where they are recorded and at least partially processed, with the aim of building up a representation of the earth strata in the area being surveyed.
In such streamer arrays, each streamer may be towed by means of its own lead-in cable, that is, an armoured electrical cable which supplies power to and receives digital signals from the streamer. Using this method, it is, typically, possible to tow a 700 metre wide array of eight streamers, each 4000 metres long.
The drag produced by such an array at a towing speed of five knots is about 40-45 tonnes, a high proportion of which is cross-line drag due to the transversely extending lead-in cables rather than in-line drag due to the streamers themselves. This drag is a very significant factor in the operating costs associated with such surveys, contributing primarily to fuel costs associated with the towing vessel.
To increase the efficiency of marine surveys of this kind, it would be desirable to use even wider streamer arrays containing a larger number of streamers. However, using current towing techniques, an array 1440 metres wide including ten streamers, for example, would produce a drag of over 70 tonnes, which makes the use of such wider arrays containing more streamers unattractive.